The island’s geography comprises relatively low-lying mountains surrounding a central plain, with several navigable rivers extending inland. The island has lush vegetation, a product of its mild but changeable climate which avoids extremes in temperature. Thick woodlands covered the island until the Middle Ages. As of 2013, the amount of land that is wooded in Ireland is about 11% of the total, compared with a European average of 35%. There are 26 extant mammal species native to Ireland. The Irish climate is very moderated and classified as oceanic As a result, winters are milder than expected for such a northerly area. However, summers are cooler than those in Continental Europe. Rainfall and cloud cover are abundant.